An epoxy resin composition has been used in a wide range of areas such as an adhesive agent, a sealing agent and a coating material. In a conventional epoxy resin composition, although a two-component epoxy resin consisting of a base resin and a curing agent has been much more common (non-patent document 1), it has been pointed out that a workability is poor since a blending operation of a base resin with a curing agent is required. Therefore, in recent years, a one-component curable epoxy resin composition excellent in workability has become more common wherein the blending operation of a base resin with a curing agent is not required since a latent curing agent is blended with an epoxy resin from the beginning.
Most of latent curing agents used for the one-component curable epoxy resin composition are in the form of particle of several micro meter and are solid dispersing type latent curing agents that are used by dispersing in a liquid epoxy resin. For examples, the solid dispersing type latent curing agents are microcapsulated materials obtained by microencapsulating a curing compound such as a dicyandiamide, a dihydrazide compound, an amine adduct compound and a amine or imidazole etc. (non-patent document 1). Since these curing compounds are separated from the epoxy resin by a microcapsule at ordinary temperatures, a curing reaction does not occur, however, a curing reaction of epoxy resin occurs by heating since the curing compound and the epoxy resin are solved each other.
However, in the case of an impregnation adhesive agent using a one-component epoxy resin composition comprising the solid dispersing type latent curing agent, there was a problem that the resin permeating into the gap can not be cured uniformly, or in some cases, can not be cured at all, since only the liquid epoxy resin deeply permeates into a gap of substances to be bonded together and particles of curing agent does not sufficiently permeate into the gap.
In cases where a coating material or a coating agent using the one-component epoxy resin composition comprising a solid dispersing type latent curing agent is applied as an extremely thin layer, there was a problem that the thickness of the cured film becomes inhomogeneous, or the cured film partially contains uncured area, since the curing agent and the epoxy resin are not blended uniformly in some areas.
In order to solve such a problem, a perfect liquid one-component epoxy resin composition has been desired wherein a solid curing agent component is not contained and an epoxy resin and a curing agent are compatible. As such a perfect liquid epoxy resin composition, a composition using an amine complex of boron trifluoride as a curing agent is disclosed (non-patent document 2). However, this resin composition generates a corrosive hydrogen fluoride gas when the curing reaction occurs, therefore, a use application thereof has been limited.
In addition, a one-component epoxy resin composition is disclosed wherein an onium salt such as sulfonium salt and pyridinium salt is used as a curing agent (non-patent document 1). However, since such a curing agent causes a cationic polymerization, there are problems in that an adhesive power is poor and an additive agent capable of being added to a resin composition has a lot of limitations.
Furthermore, a one-component epoxy resin composition, wherein an amine imide compound is used as a latent curing agent, is disclosed (patent document 1). However, there was a problem that not only a curing speed of this composition was remarkably slow but also a curing temperature was high, therefore, a severe curing condition of 150° C. for 3 hours was required to obtain a practical adhesive strength.
On the other hand, a composition composed by an epoxy compound having two or more epoxy groups, a polythiol compound having three or more thiol groups and an amine, which enables to work as an impregnation adhesion or to form thin sheet-like cured material, was disclosed as an epoxy resin composition wherein not only the solid dispersing type latent curing agent but also a curing accelerator was not used (patent document 2). However, since the storage stability of this composition at ordinary temperatures, is extremely low, it cannot be used practically.
Moreover, a composition composed by a multifunctional epoxy compound, a multifunctional thiol, an amine and a mercapto organic acid as a curing retardant was disclosed (patent document 3), however, curing properties of this composition were also insufficient.
In addition, a thiol epoxy resin composition having a good storage stability and excellent curing properties was disclosed, wherein a polythiol compound which had two or more thiol groups in a molecule, was used as a curing agent, and also a reactant obtained by reacting a compound having one or more isocyanate group in a molecule with a compound having at least one primary and/or secondary amino group in a molecule, was used as a curing accelerator (patent document 4).
However, this composition did not always have an excellent reproducibility and was not a practical composition, since a curing agent etc. were not dissolved homogeneously in a resin composition, and as a result, uncured parts were sometimes found. Also, an isocyanate compound, which gives excellent curing properties to an epoxy resin composition, has extremely high toxicity. Therefore, an epoxy resin composition that does not contain an isocyanate compound has been desired from the viewpoint that an isocyanate compound is a dangerous compound having negative effects on the environment and a person's health.
On the other hand, as an epoxy resin composition having further improved storage stability, an epoxy resin composition, comprising an epoxy resin that has two or more epoxy groups in a molecule, a thiol compound that has two or more thiol groups in a molecule, a solid dispersing type latent curing accelerator and a boric acid ester compound as an essential component, is disclosed (patent document 5).
However, as described in the specification that “it is thought that the boric acid ester compound reacts with the surface of the solid dispersing type latent curing accelerator to decorate and encapsulate the accelerator” (See, the beginning of [0025], patent document 5), the boric acid ester compound which is added in order to further improve the storage stability has been limited to the case wherein the solid dispersing type curing accelerator is used.
Thus, a one-component curable resin composition, which not only has excellent quick curing properties and an excellent storage stability, but also is able to be cured uniformly without having uncured parts when it is formed into an extremely thin cured coating film, or when it is permeated into a narrow gap, has not been known yet.